1. Field of the Invention
The present invention relates to a method and apparatus that reduce warping of an optical disc such as a DVD or compact disc and, particularly, to a method and apparatus that effectively cool while simultaneously reducing warping in an injection molded disc substrate.
2. Description of the Related Art
Generally, an optical disc substrate such as a DVD substrate or a compact disc substrate is molded by injection molding. As is shown in FIG. 8A, when molding a disc substrate, a resin material that is to become a disc substrate is injected into a pair of molds 10 and 11 so as to form a molded disc substrate 1. At this time, predetermined information is recorded on one surface of the molded disc substrate 1. Next, as is shown in FIG. 8B, the molds 10 and 11 are opened on both sides, and a takeout arm 12 of a takeout mechanism (not shown) moves forward between the molds 10 and 11, and holds the molded disc substrate 1, which is still soft and at a high temperature, by suction. The takeout arm 12 then moves backwards and removes the molded disc substrate 1 from the molds 10 and 11.
Next, as is shown in FIG. 8C, the disc substrate 1 that has been taken out of the molds 10 and 11 is held directly by suction by a transporting arm 13 of a transporting mechanism in a particular position, and is then transported by the transporting arm to a cooling stage 14. More specifically, the molded disc substrate 1 that has been removed from the molds 10 and 11 and is held by suction by an suction portion 12a of the distal end of the takeout arm 12, has a first surface and a second surface on the opposite side to the first surface that is held by the suction of the takeout arm 12. This second surface is held by suction by a holding portion 13a provided at the distal end of a transporting arm 13. Practically simultaneously with this, the suction portion 12a of the takeout arm 12 releases its hold of the molded disc substrate 1. Thereafter, while holding it the transporting arm 13 turns the molded disc substrate 1 by substantially 180 degrees, and then transfers the molded disc substrate 1 to the cooling stage 14. The molded disc substrate 1 is then cooled by the cooling stage 14, and the resin material thereof is solidified so as to form a disc substrate 1. The temperature at which this solidifying may take place is, for example, approximately 90° C.
As described above, immediately after being taken out from the molds of the molding machine the molded disc substrate 1 has a high temperature and is fairly soft. Therefore, in the process in which it is cooled and the resin material thereof is solidified so as to form the disc substrate 1, it can be understood that the molded disc substrate undergoes considerable deformation, namely, warping. Conventionally, in order to reduce this warping, various conditions are adjusted during the injection molding, for example, the warping may be reduced by adjusting the temperature of the molds, or by setting different temperatures for the mold 10 and the mold 11, alternatively, the pressure may be adjusted when the resin material is being injected.
However, even if a molded disc substrate 1 with a small amount of warping is obtained through the methods described above, because immediately after being taken out from the molds of the molding machine the molded disc substrate 1 has a high temperature and is in a sufficiently soft state, the problem arises that the molded disc substrate 1 becomes warped when it is taken out from the molds, and also during the process of being moved by the transporting mechanism to the cooling stage. Moreover, there is a large amount of warping even during the cooling process. This point is described below using FIGS. 9A to 9C.
FIG. 9A shows the suction portion 12a of the takeout arm 12 holding the molded disc substrate 1 in order to remove the disc substrate 1 from the molds of the molding machine, or alternatively shows the suction holding portion 13a of the transporting arm 13 holding the molded disc substrate 1 by suction. Because the molded disc substrate 1 has a high temperature and is in a sufficiently soft state, it has been confirmed that when an arm adheres to the center portion of the molded disc substrate 1 then that molded disc substrate 1 is bent by the force of the suction in the opposite direction from the side of the suction. In particular, because the thickness of a DVD is approximately a thin 0.6 mm as compared with the 1.2 mm thickness of a compact disc, the bending in a DVD is far more marked.
Moreover, as is shown in FIG. 9B, it has also been confirmed that if the takeout arm 12 or the transporting arm 13 turns or moves at a comparatively fast speed while holding by suction a molded disc substrate 1, then the molded disc substrate 1 is warped by the wind pressure of that action.
Furthermore, as is shown in FIG. 9C, it has further been confirmed that there are cases when a molded disc substrate 1 is warped by its own weight during the period it is being transferred to a disc receiving base of the cooling stage 14 and is cooling. In addition, there are also cases in which the molded disc substrate 1 is partially deformed by the structure of the cooling stage when the molded disc substrate 1 is cooling on the cooling stage.
The above warping is generated not only in the radial direction of the molded disc substrate 1 (that is, the substrate 1 warps in a saddle shape), but also in the tangential direction of the disc substrate 1 (that is, the substrate 1 is warped in a wavy pattern at the periphery thereof). This warping appears more marked as the disc substrate production cycle time is shortened, so that the problem thereby arises of the warping in the disc substrate increasing.
In order to solve the above described conventional problems, by applying centrifugal force by rotating a molded disc substrate at high speed while the molded disc substrate was still soft before the resin material thereof had solidified, so as to reduce warping and deformation in the disc substrate while simultaneously cooling the disc substrate, the present inventors were able to obtain a disc substrate with only a small amount of warping and deformation, as is described in Japanese Patent Application No. 2001-197738 filed on Jun. 29, 2001.
It is an object of the present invention to solve the problem of how to further reduce warping in the molded disc substrate 1 by supplying cooling gas to the molded disc substrate 1 while the molded disc substrate 1 is placed on a disc receiving base 2 and is being rotated at high speed so as to cool the molded disc substrate 1 to room temperature in a short time and thereby achieve a shortening in the production time. At the same time, it is an object of the present invention, either while the rotation speed is being accelerated or once the molded disc substrate 1 is rotating at full speed, to prevent flapping, which occurs when the cooling gas is being blown onto the molded disc substrate 1 as it rotates, in order to reduce the warping of the disc substrate.